1. Field of the Invention
The invention relates to coated inorganic pigments, to a process for their preparation by the sol-gel process and to their use for the production of enamels and mouldings.
2. Description of the Background
It is already known to apply one-component (e.g. SiO.sub.2) or multi-component glass coats to planar substrates by the sol-gel process; see, for example, DE 3719339, DE 4117041 and DE 4217432. Also, it is known to provide inorganic particles with ceramic coatings by means of the sol-gel process. For example, SiO.sub.2 particles can be coated with a ZrO.sub.2 layer, or copper chromium spinel particles with a layer of mullite. The layer thickness of the coating in these cases is within the nm range up to about 100 nm. However, thin layers of this kind are unsuitable for protecting redox-sensitive inorganic pigments against oxidation or reduction processes at temperatures in the range from 500 to 700.degree. C., as are employed for the production of enamels, since these layers are unable effectively to suppress the diffusion of gaseous substances. In addition, the very thin coatings break up within a short time in the presence of aggressive glass melts, as are used, for example, for enamel decorations on glassware, and as a result the pigment decomposes, with a change in colour, or is dissolved in the glass matrix.